1. Field of the Invention
This invention relates to a power supply circuit for biasing a field-effect transistor amplifier and, more particularly, to a power-supply circuit for biasing a GaAs (gallium arsenide) field-effect transistor amplifier with a single power supply in the same way as with a biasing system employing both positive and negative power supplies to raise efficiency and obtain higher reliability.
2. Related Art
In general, when use is made of a GaAs field-effect transistor (hereinafter referred to as a "FET"), a positive voltage must be applied across a drain and a source of the transistor after a negative voltage is applied across a gate and the source of the transistor. Further, a GaAs FET requires a negative power supply.
Accordingly, in order to operate a GaAs FET with a single power supply, a source resistor is inserted between the source and a ground and the gate potential is set lower than the source potential to obtain an effect that is equivalent to the application of a negative voltage to the gate. With this biasing method, however, loss is produced owing to the source resistor and a decline in gain is incurred as a result.
In addition, in the case of a power GaAs FET for generating a high-power output, an arrangement is available in which the source is grounded to a case within the device to strengthen a ground pass and improve the heat radiating property. However, this structure prohibits the insertion of the source resistor.
Since most semiconductor devices presently manufactured are so designed as to operate using a single power supply system, a circuit system for biasing the GaAs FET amplifier with a single power supply is needed in order to achieve conformity with such devices.
One of conventional methods for biasing a GaAs FET, uses both positive and negative power supplies. As shown in FIG. 1, this conventional method relies upon drain biasing, in which a positive voltage from a positive power supply 6 is applied to the drain of a GaAs FET 3 through a choke coil 1, as well as gate biasing, in which a negative voltage from a negative power supply 7 is applied to the gate of the GaAs FET 3 through a choke coil 2. The source of the GaAs FET 3 is grounded. The drain and gate biasing voltages are blocked by coupling capacitors 4 and 5, respectively, so as not to apply the voltages to the outside.
Another conventional biasing method is biasing a GaAs FET by a single power supply, as illustrated in FIG. 2. According to this method, drain bias from a positive power supply 6 is applied to the drain of the GaAs FET 3 via the choke coil 1, the gate is grounded via the choke coil 2 and the source is grounded via a source resistor 8. The drain and gate biasing voltages are blocked by the coupling capacitors 4 and 5, respectively, so as not to apply the voltages to the outside.
GaAs FETs having excellent characteristics in the microwave band have been developed and are finding widespread use. Recently, with the development of the quasi-microwave band in mobile communications and the like, utilization of GaAs FETs in this band has shown great promise. In order to utilize a GaAs FET in mobile communications, both low power consumption and high efficiency are required. Further, such a function as to allow the power supply for amplifiers to be cut off when unnecessary is desirable in order to conserve the battery.
With the conventional biasing system employing two power supplies shown in FIG. 1, the efficiency of the amplifier is high but both positive and negative voltages are required as the biasing voltages. With regard to the circuit elements other than the amplifier, almost all of them are operated by a single power supply of +5V or +3V. Accordingly, this biasing system which requires both positive and negative power supplies is not effective in consideration of the coexistence with the circuits operating on a single power supply.
With the conventional biasing system shown FIG. 2, employing one power supply, only the positive biasing power supply suffices. However, since the source resistor 8 is inserted between the source and ground, the amount of loss is large. If the GaAs FET is utilized as an amplifier, the source resistor 8 deteriorates the gain of the amplifier to reduce efficiency. Moreover, in recently designed and manufactured high-power devices, since the source is grounded to a case within the device in order to enhance the capability for heat radiation and to strengthen the ground pass, the source resistor cannot be inserted.
Further, when bias is applied to a GaAs FET, it is necessary to apply the gate bias before applying the drain bias. Since the input impedance of a GaAs FET is very high, applying the drain bias before the gate bias may result in gate-source biasing owing to electrostatic induction or the like. This can result in the GaAs FET being driven into conduction and can lead to destruction of the device in a worst case.
With the above-described conventional biasing system, employing a single power supply is such that the gate bias is not applied unless the drain bias is applied. As a consequence, the device may likely be destructed in applications where the power supply is turned on and off frequently.
If the above-described timing sequence for biasing the GaAs FET is to be implemented in the conventional biasing system that relies upon two power supplies controlling the timing at which bias is applied is complicated enlarges the circuit size.